A new concept for isotope ratio monitoring liquid chromatography/mass spectrometry

A new interface for the on-line coupling of a liquid chromatograph to a stable isotope ratio mass spectrometer has been developed and tested. The interface is usable for (13)C/(12)C determination of organic compounds, allowing measurement of small changes in (13)C abundance in individual analyte species. All of the carbon in each analyte is quantitatively converted into CO(2) while the analyte is still dissolved in the aqueous liquid phase. This is accomplished by an oxidizing agent such as ammonium peroxodisulfate. The CO(2) is separated from the liquid phase and transferred to the mass spectrometer. It is shown that the whole integrated process does not introduce isotope fractionation. The measured carbon isotope ratios are accurate and reproducible. The sensitivity of the complete system allows isotope ratio determination down to 400 ng of compound on-column. By-passing the high-performance liquid chromatography (HPLC) separation allows bulk isotopic analysis with substantially lower sample amounts than those required by conventional elemental analyzers. The results of the first applications to amino acids, carbohydrates, and drugs, eluted from various types of HPLC columns, are presented. The wide range of chromatographic methods enables the analysis of compounds never before amenable to isotope ratio mass spectrometry techniques and may lead to the development of many new assays.     

Isotope ratio monitoring gas chromatography/Mass spectrometry of D/H by high temperature conversion isotope ratio mass spectrometry

Of all the elements, hydrogen has the largest naturally occurring variations in the ratio of its stable isotopes (D/H). It is for this reason that there has been a strong desire to add hydrogen to the list of elements amenable to isotope ratio monitoring gas chromatography/mass spectrometry (irm-GC/MS). In irm-GC/MS the sample is entrained in helium as the carrier gas, which is also ionized and separated in the isotope ratio mass spectrometer (IRMS). Because of the low abundance of deuterium in nature, precise and accurate on-line monitoring of D/H ratios with an IRMS requires that low energy helium ions be kept out of the m/z 3 collector, which requires the use of an energy filter. A clean mass 3 (HD(+.)) signal which is independent of a large helium load in the electron impact ion source is essential in order to reach the sensitivity required for D/H analysis of capillary GC peaks. A new IRMS system, the DELTA(plus)XL(trade mark), has been designed for high precision, high accuracy measurements of transient signals of hydrogen gas. It incorporates a retardation lens integrated into the m/z 3 Faraday cup collector. Following GC separation, the hydrogen bound in organic compounds must be quantitatively converted into H(2) gas prior to analysis in the IRMS. Quantitative conversion is achieved by high temperature conversion (TC) at temperatures >1400 degrees C. Measurements of D/H ratios of individual organic compounds in complicated natural mixtures can now be made to a precision of 2 per thousand (delta notation) or, better, with typical sample amounts of approximately 200 ng per compound. Initial applications have focused on compounds of interest to petroleum research (biomarkers and natural gas components), food and flavor control (vanillin and ethanol), and metabolic studies (fatty acids and steroids). Copyright 1999 John Wiley & Sons, Ltd.     

Extended-ion approximation and pressure shifts inF-band energies

Pressure variation of maximumF-band absorption energies in the halides of lithium, sodium and potassium has been investigated employing the extended-ion approximation for the calculation of theF-electron energy eigenvalues and using values of local compressibility in the neighbourhood of theF-centres which include the effect of vacancy and pressure. The results obtained agree with the experimental results.     

Mass spectrometric method for the absolute calibration of the intramolecular nitrogen isotope distribution in nitrous oxide

A mass spectrometric method to determine the absolute intramolecular (position-dependent) nitrogen isotope ratios of nitrous oxide (N₂O) has been developed. It is based on the addition of different amounts of doubly labeled ¹⁵N₂O to an N₂O sample of the isotope ratio mass spectrometer reference gas, and subsequent measurement of the relative ion current ratios of species with mass 30, 31, 44, 45, and 46. All relevant quantities are measured by isotope ratio mass spectrometers, which means that the machines inherent high precision of the order of 10^–5 can be fully exploited. External determination of dilution factors with generally lower precision is avoided. The method itself can be implemented within a day, but a calibration of the oxygen and average nitrogen isotope ratios relative to a primary isotopic reference material of known absolute isotopic composition has to be performed separately. The underlying theoretical framework is explored in depth. The effect of interferences due to ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O in the ¹⁵N₂O sample and due to ¹⁵N ₂ ⁺ formation are fully accounted for in the calculation of the final position-dependent nitrogen isotope ratios. Considering all known statistical uncertainties of measured quantities and absolute isotope ratios of primary isotopic reference materials, we achieve an overall uncertainty of 0.9 (1). Using tropospheric N₂O as common reference point for intercomparison purposes, we find a substantially higher relative enrichment of ¹⁵N at the central nitrogen atom over ¹⁵N at the terminal nitrogen atom than measured previously for tropospheric N₂O based on a chemical conversion method: 46.3±1.4 as opposed to 18.7±2.2. However, our method depends critically on the absolute isotope ratios of the primary isotopic reference materials air–N₂ and VSMOW. If they are systematically wrong, our estimates will also necessarily be incorrect.     

Synthesis, structure, and physical properties for a series of trigonal bipyramidal M(II)-Cl complexes with intramolecular hydrogen bonds

A series of transition metal chloro complexes with the tetradentate tripodal tris(2-amino-oxazoline)amine ligand (TAO) have been synthesized and characterized. X-Ray structural analyses of these compounds demonstrate the formation of the mononuclear complexes [M(II)(TAO)(Cl)](+), where M(II) = Cr, Mn, Fe, Co, Ni, Cu and Zn. These complexes exhibit distorted trigonal-bipyramidal geometry, coordinating the metal through an apical tertiary amine, three equatorial imino nitrogen atoms, and an axial chloride anion. All the complexes possess an intramolecular hydrogen-bonding (H-bonding) network within the cavity occupied by the metal-bound chloride ion. The metal-chloride bond distances are atypically long, which is attributed to the effects of the H-bonding network. Nuclear magnetic resonance (NMR) spectroscopy of the Zn complex suggests that the solid-state structures are representative of that observed in solution, and that the H-bonding interactions persist as well. Additionally, density functional theory (DFT) calculations were carried out to probe the electronic structures of the complexes.